1. Field of the Invention
This invention relates to nuclear reactors and, more particularly, to hydraulic apparatus for absorbing shocks that are applied to fuel assemblies, and the like.
2. Description of the Prior Art
To produce useful power from a nuclear reactor, it is necessary to assemble fissionable uranium in a sufficient concentration and in a physical configuration that will sustain a continuous sequence of neutron-induced fission reactions within the uranium nuclei. The heat generated through these reactions in this assembly, or reactor core, usually is absorbed in a stream of pressurized water. This heated pressurized water then is pumped to one or more heat exchangers in which the absorbed heat is transferred to secondary coolant water. It is, of course, this secondary coolant water that rises into the steam which drives the turbines, or other electrical power generating machinery.
To provide a proper concentration of uranium for the reactor core, it has often been the practice to prepare pellets of uranium dioxide. These pellets are loaded into long, slender, hollow tubes which, when the tube ends are sealed off, are referred to as fuel rods. In order to enhance the structural integrity of the reactor core, these fuel rods are arranged into subgroups, each of about two hundred fuel rods, that are called fuel assemblies. The assemblies, in turn, are mounted in a generally right circular cylindrical array to form the reactor core.
Naturally, the reactor core is environmentally hostile to the structural integrity of its component parts. The temperature, water flow velocity, pressure, radiation and the like within the reactor core all combine to place great stresses on the core materials. In addition to these environmental extremes, adequate provision also must be made to enable structural components of the reactor core to cope with other forces of a more unusual and, perhaps, of a more violent nature than those which are imposed through ordinary operating conditions. Seismic or earthquake shocks and the thermal shocks to physical structure that might attend an accident in which a significant portion of the pressurized water evaporates from or drains out of the reactor core are typical of the situations in which forces far in excess of those generated in the course of routine operation could be encountered.
The customary response to this problem is the addition, in one way or another, of more materials and more metal to the reactor core. This direct approach although probably providing the needed structural protection, has a number of undesirable features. Additional materials in the reactor core, for example, exhibit a "parasitic" effect that absorbs a portion of the neutron population within the core. Neutrons, absorbed in this manner do not contribute to the energy production and hence, are used wastefully and inefficiently.
Accordingly, there is a need for improvements to reactor core structures that will enable the core to safely attenuate or absorb shocks and other forces of unusual and major character without adding materials to the core structure that will not increase parasitical neutron losses.